Batch message-board subscriber reads into one SQLite result (#47050)

## Why

Posting to the local agent message board fetched each subscriber row across SQLite's worker thread while holding a write transaction.

## What changed

Use `json_group_array(agent)` to fetch subscribers as a single JSON array, then deserialize it before collecting notification recipients.

## Testing

Extend the local board test to verify that posting to a channel with no subscribers succeeds without sending notifications.

GitOrigin-RevId: ad294e19c78ee501f1de2084f7c02fa96f53418d

codex-rs/ext/agent-message-board/src/local.rs

@@ -248,14 +248,17 @@ impl LocalAgentMessageBoard {
                 )
             }
         };
+        // Return one JSON array to avoid handing each subscriber row across
+        // SQLite's worker thread while the write transaction is held.
         let subscribed = sqlx::query_scalar::<_, String>(
-            "SELECT agent FROM subscriptions WHERE board=? AND target=?",
+            "SELECT json_group_array(agent) FROM subscriptions WHERE board=? AND target=?",
         )
         .bind(self.identity.to_string())
         .bind(target_key(&target)?)
-        .fetch_all(&mut *tx)
+        .fetch_one(&mut *tx)
         .await
         .map_err(storage_error)?;
+        let subscribed: Vec<String> = serde_json::from_str(&subscribed).map_err(storage_error)?;
         for recipient in subscribed {
             recipients.insert(ThreadId::from_string(&recipient).map_err(storage_error)?);
         }

codex-rs/ext/agent-message-board/tests/local_board.rs

@@ -165,6 +165,23 @@ async fn shared_handles_resume_posts_and_preserve_subscription_rules() {
         )
         .await
         .unwrap();
+    // Posting still succeeds when the subscriber lookup returns an empty array.
+    resumed
+        .post(
+            root,
+            PostRequest {
+                request_id: "no-subscribers".into(),
+                destination: PostDestination::Channel("proofs".into()),
+                text: "saved without notifications".into(),
+                agents_to_notify: Vec::new(),
+            },
+        )
+        .await
+        .unwrap();
+    assert_eq!(
+        *host.notifications.lock().unwrap(),
+        vec![(child, metadata.clone())]
+    );
     resumed
         .set_subscription(
             root,